1. Field of the Invention
The present invention relates generally to switched-mode power supply circuits, and more particularly to startup circuits for such switched-mode power supply circuits.
2. Description of the Related Art
Electronic products usually require power supply systems that take power from a high-voltage source and deliver regulated power either directly to the product or to a battery that serves as the source of power for the product. Today, these power supplies are typically switched-mode power supply (SMPS) circuits. These SMPS systems generally incorporate an integrated circuit (IC) which controls the regulation of the delivered power by switching one or more transistors connected to a transformer or inductor. These SMPS Control IC's are generally designed so that when they are operating and delivering power to the load, they simultaneously deliver power to the IC itself. This is done to reduce power loss and improve the efficiency of the system, by powering the SMPS Control IC off a low-voltage source rather than off a high-voltage source. Since the low-voltage source is generated by the SMPS Control IC, it is not available until the SMPS Control IC starts to switch the transformer or inductor. This results in the common need for a startup circuit (sometimes called a “bootstrap” circuit) as a mechanism to deliver startup current to the SMPS Control IC directly from the high-voltage source until the SMPS Control IC starts to operate or “wakes up”. To reduce the amount of power directly required from the high-voltage source, SMPS Control IC's are usually designed to have very low current consumption until they wake up and start delivering power to themselves. This is often referred to as the SMPS Control IC having a sleep current consumption that is much lower than the operating current consumption.
The simplest and lowest cost startup circuit is a resistor between the high-voltage source and the power supply or VDD pin of the SMPS Control IC. This resistor has the following two key disadvantages: the startup current depends on the voltage of the high-voltage source; and the startup circuit is a source of continuous power loss.
The prior art includes a variety of circuits in which transistors are connected between the high-voltage source and the VDD pin of the SMPS Control IC, and these transistors are switched off once the SMPS Control IC is operational.